Fiber laser

ABSTRACT

A fiber laser with a fiber ( 16 ) for laser light generation having an entrance end ( 14 ) and an exit end ( 20 ) comprises a pump light source ( 10 ) for generating pump light to be coupled via the entrance side ( 14 ) into the fiber ( 16 ). At the exit end of the fiber ( 16 ) a first resonator mirror ( 26 ) is provided which is highly reflecting for the laser light to be generated in the wavelength range with the smallest light amplification and to the light of the pump light source ( 10 ). Spaced from the first resonator mirror ( 26 ) a second resonator mirror ( 28 ) is provided via which light of further wavelength ranges can be fed back into the fiber ( 16 ) with the aid of a collimating lens ( 30 ).

[0001] The invention relates to a fiber laser having a doped opticalfiber for laser light generation.

[0002] In a fiber laser the laser-active medium is incorporated in alight waveguide. Laser activity of the fiber is attained in particularby doping the fiber core with ions of rare earths. In numerous lasertransitions of rare earth ions it was possible for the first time toobserve laser emission in fiber lasers, in particular since it has beenpossible to use, besides silicate glasses, fluorite glasses, above allfluor zirconate glass ZBLAN, as a host material. In contrast to silicateglasses fluorite glasses have smaller effective photon energies, whichresults in smaller rates of non-radiative decay, longer durations ofexcited states and larger amplification cross-sections. Fluorite glassesallow high quantum efficiencies and an efficient excitation intoelevated energy levels to be attained by absorption out of excitedstates such that laser emission out of states, in which the excitationenergy is larger than the quantum energy of the exciting light (upwardconversion laser), is possible.

[0003] Active fibers can generally be produced by doping the fiber corewith laser-active materials. Here, excitation of the ions is effectedvia a pump light source for generating pump light to be coupled into thefiber. The pump light is longitudinally irradiated into the fiber suchthat said light is absorbed by the ions. The pump light is focussed,with the aid of a lens, onto the front side of the fiber, coupled intothe fiber core and guided there.

[0004] If only a portion of the light coupled into the fiber is absorbedby the corresponding ions, a high-performance pump light source isrequired for generating an adequate laser intensity. For increasing theefficiency of the laser, i.e. in particular of the amount of absorbedlight from the pump light source, it is common practice to feed backinto the fiber, with the aid of a mirror, light emerging at the exit endof the fiber, which has not yet been absorbed by the ions contained inthe fiber. Further, it is common practice to evaporate the resonatormirror directly onto the fiber end.

[0005] Such a fiber laser is e.g. known from DE 196 36 236 A1. The diodelaser-pumped multimode waveguide laser comprises a diode laser. With theaid of a collimating optics the light emitted by the diode laser iscoupled into the fiber at the entrance end of the fiber. A mirror isevaporated onto the entrance end of the fiber. The mirror reflects thepumping wavelength generated by the diode laser only to a very smallextent. However, the mirror arranged at the entrance end reflects thegenerated laser light extremely well. Thus, at the entrance end of thefiber almost only light with the pumping wavelength can emerge from thefiber. The opposite fiber end, the exit end of the fiber, can beantireflection-coated for the laser wavelength to be generated.

[0006] For feeding laser light generated in the fiber back into thefiber, a mirror is arranged in a spaced relationsship to the exit end ofthe fiber. The light reflected by this resonator mirror is focussed withthe aid of a lens disposed between the exit end of the fiber and theresonator mirror and fed back into the fiber.

[0007] It is an object of the invention to provide a fiber laser whichallows emission at a plurality of wavelengths. Preferably, the fiberlaser is adapted to emit a plurality of wavelengths simultaneously or ina selective and switchable manner. Further, it should preferably bepossible to use a low-performance pump light source in connection withthe fiber laser.

[0008] According to the invention, this object is achieved with thefeatures of claim 1.

[0009] The fiber laser according to the invention comprises a fiber forlaser light generation having an entrance end and an exit end. Inparticular, this is an active fiber doped with rare earth ions. Forcoupling pump light via the entrance end into the fiber, a pump lightsource is provided. The pump light source is e.g. a laser diode. Thelaser diode preferably generates light with an excitation wavelength oran excitation wavelength range. Further, a resonator unit is provided atthe exit end of the fiber for feeding, preferably frequency-selectively,light emerging from the fiber back into the fiber. According to theinvention, the resonator unit comprises a first and a second resonatormirror. The first resonator mirror is directly connected with the exitend of the fiber. Said connection can e.g. be realized by depositing themirror. The second resonator mirror is arranged in a spaced relationshipto the exit end of the fiber.

[0010] Arrangement of two or more resonator mirrors in a resonator unitat the exit end of the fiber allows laser emission at two or morewavelengths. The first resonator mirror connected with the exit end ofthe fiber is preferably configured such that it reflects the desiredwavelength back into the fiber with the smallest light amplification.With the aid of the second spaced resonator mirror another selectedwavelength can then be fed back into the fiber. In this manner, thewavelengths of the light generated by the fiber laser are selected.

[0011] The first resonator mirror, which is preferably glued to the exitend of the fiber, is preferably highly reflecting for the generatedlaser light in the wavelength range with the smallest lightamplification and in the range of the pump light. In particular, thereflectivity for the wavelength range of the laser emission amounts to50 to 99.9%. Particularly preferred is a reflectivity of 80% to 99.1%.As a result, 0.1% to 50%, preferably 0.9% to 20% of the generated laserlight can be coupled out. For the pump wavelength the reflectivitypreferably amounts to more than 50%, in particular more than 75%. Such amirror allows laser emission at the weakest transition with the pumplight being well utilized.

[0012] The second resonator mirror is preferably highly reflecting foranother wavelength range. It is thus possible to feed further radiationemitted by the ions back into the fiber and utilize said radiation forexciting the laser emission at additional wavelengths.

[0013] Further resonator mirrors may be provided which are highlyreflecting for a certain wavelength range and are transparent to or havean extremely low reflectivity in the remaining wavelength ranges. Withthe aid of such resonator mirrors certain wavelength ranges can thus becontrolledly fed back into the fiber. The resonator involved is thus aspectrally staggered multiple resonator. This allows the light losses atthe laser transition with the smallest amplification to be minimizedand/or the required pumping power to be reduced. The spectrallyselective feedback of light by the further mirrors ensures the start ofoscillation of the laser oscillator at the other desired wavelengths orwavelength ranges. The invention allows switchable or simultaneousemission at a plurality of laser transitions using a low-performancepump light source, e.g. a laser diode. Another method for generatingsimultaneous or switchable light emission at a plurality of wavelengthsis to controlledy position the first resonator mirror preferably at adistance of up to 3 μm to the end face of the glass fiber. The change inthe air gap thus created allows a controlled modification of thespectrum of the fed-back light.

[0014] Since the laser light generated by the fiber depends on thewavelength range of the fed-back light, mirrors reflecting differentwavelength ranges can be used or exchanged. Preferably, a diaphragmand/or color filter for controlling the wavelength range of the laserlight to be generated is arranged upstream of the second or everyfurther resonator mirror. Such an arrangement ensures that e.g. onlylight with a certain wavelength or a certain wavelength range is fedback by the mirror. It is thus also possible to provide a resonatormirror which is reflecting for a plurality of wavelength ranges, and toselect, with the aid of suitable diaphragms, e.g. only one of thesecolor ranges. An adequate control of the diaphragm, the color filterand/or the mirror spacing allows laser light to be generatedsimultaneously in at least two or more wavelength ranges. This can e.g.be realized by partially covering the resonator mirror. Thus the ratioof the light power of individual wavelength ranges can be set.

[0015] Preferably, the resonator unit comprises a collimating lens forsetting the wavelength range fed back into the fiber, said collimatinglens being longitudinally shiftable relative to the exit end of thefiber. Shifting of the collimating lens allows individual wavelengthranges reflected by the resonator mirror or the resonator mirrors to becoupled into the fiber. The collimating lens can preferably be used insuch a way that it serves for controlling the emission in thecorresponding wavelength ranges.

[0016] Preferably, a coupling-in mirror transparent to the wavelengthrange of the pump light is provided at the entrance end of the fiber. Toallow setting of the efficiency of the radiation amplification in therange of the desired emission wavelength of the fiber, the coupling-inmirror offers, on the side facing the entrance end of the fiber, anadequately high reflectivity to the emission wavelength range of thefiber laser.

[0017] Due to the feedback of a portion of the light from the pump lightsource the properties of the pump light source and thus those of thefiber laser may change. The wavelength range of the pump light isreflected in particular by the first resonator mirror towards theentrance end. The light reflected by the first resonator mirror is notcompletely absorbed by the ions contained in the fiber. Since thecoupling-in mirror at the entrance end of the fiber is transparent tothe pumping wavelength, the portion which is not absorbed emerges at thecoupling-in mirror in the direction of the pump light source, e.g. thelaser diode. This may lead to a modification of the spectrum of thelaser diode and, in the case of strong back-reflections due to excessivepower increase at the front face of the laser diode, to a reducedservice life of the laser diode. Back-reflections further occur at theentrance end of the fiber since the entrance end is not perfectlyantireflection-coated such that reflections occur. All types ofreflection may not only lead to the described disturbances in the pumplight source, such as the laser diode, but also to disturbances in otherparts of the fiber laser. For example, a photodiode is normallyinstalled in the housing of the laser diode, said photodiode serving asa power monitor of the fiber laser. The signal of this photodiode willbe falsified by the back-reflections.

[0018] The aforementioned problem can be solved by an optical insulationor an optical decoupling of the pump light source from the entrance endof the laser. It is common practice to use so-called Faraday insulatorsfor this purpose.

[0019] If the fiber does not show any birefringence, an opticalinsulator can preferably be used, said insulator comprising a polarizingfilter and a λ/4 plate by means of which the linearly polarized beam iscompletely circularly polarized. During another passage of a reflectedbeam through the λ/4 plate towards the pump light source the beam istransformed again. Thereby a linearly polarized beam is created which isrotated by 90° relative to the beam originally emitted by the pump lightsource. Such a beam can no longer pass through the polarizing filter.

[0020] Preferably, in the fiber laser according to the invention thefirst resonator mirror and/or the coupling-in mirror are provideddirectly at the corresponding fiber end. For example, it is possible toevaporate a corresponding specular layer onto the fiber end.

[0021] Prior to arranging the mirror, e.g. by coating the fiber ends,the fiber ends are preferably enclosed with a material of similarhardness and then polished together with said material. Due to thesimilar, ideally identical, hardness of the fiber and the materialenclosing the fiber an extremely flat plane can be produced. Thematerial enclosing the fiber end is preferably a plastic material whichpolymerizes when exposed to UV-radiation.

[0022] Evaporation of mirrors onto the fiber front faces is however acomplicated process. In particular reproduction of numerous identicalmirrors is extremely complicated.

[0023] An alternative method for producing an extremely flat plane onthe fiber front face is to break the fiber, in particular with the aidof a fiber cutting means.

[0024] According to the invention, the mirrors arranged on a carriermaterial are preferably glued to the fiber ends, i.e. to the entranceend and/or the exit end of the fiber. Such glueing of mirrors to thefiber ends of a laser is an independent invention. For example, glueingof mirrors, as described below, can also be effected on conventionalfiber lasers or other devices comprising a fiber with aluminized ends.

[0025] Preferably, glueing is effected with a low-viscosity adhesive. Inparticular, the adhesive is a two-component epoxy resin adhesive.

[0026] Preferably, the mirror has a plurality of dielectric layers andis produced on a glass substrate, preferably in vacuum. In particular,the mirror comprises two types of layers of different materials arrangedin mutually alternating relationship. The two alternating layers arepreferably made of hafnium oxide or silicon oxide. In a particularlypreferred aspect the first resonator mirror comprises two multilayersubsystems reflecting the laser light or the pump light.

[0027] According to the invention, the fiber is preferably doped with atleast one rare earth or lanthanite. Preferably, praseodym and/orytterbium and/or erbium ions are used for doping the fiber. Doping withpraseodym ions is preferably effected in the range from 500-5000 ppm, inparticular in the range from 2000-3000 ppm. The ytterbium ions, whichmay be supplied in addition to the praseodym ions, are preferablysupplied in a quantity of 5000-50000 ppm, preferably from 10000-30000ppm.

[0028] To allow laser emission in the transverse basic modes, the fiberis preferably configured such that the cut-off wavelength for higherwaveguide modes of the fiber is by 5%-15% larger than the shortest laseremission wavelength. The cut-off wavelength characterizes the shortestwavelength which is guided by the fiber only in the transverse basicmode.

[0029] Preferably, the pump light source, which is preferably a laserdiode, is controlled by a control signal such that the emission power ofthe fiber laser can be controlled. For this purpose, a control signal isderived from the level of emission power, said signal serving forenergizing the pump light source.

[0030] A preferred control process is e.g. carried out by applying thefollowing method: After collimation of the light emerging from thefiber, a portion of the laser light (≈33%) is coupled out at apolarization-independent beam splitter. A narrow-band filter focussesthis light onto a photodiode. The photodiode signal is electronicallyamplified, inverted and differentiated. The signal thus processedmodulates the laser diode current, wherein the intensity of modulationis settable. The circuit is set up such that a portion of the constantcurrent is guided via a transistor past the laser diode. Due to use ofthe constant current source with a specified maximum current, theelectronic control unit cannot damage the sensitive laser diode.Essential components of the circuit are a photodiode, a load resistor,an amplifier, an inverting differentiator and a transistor energized byan operational amplifier.

[0031] A preferred means for the simultaneous generation of two controlsignals is shown in FIG. 7. For simultaneous two- or multicolor emissionof the laser, two or more control channels are required.

[0032] Modulation of the laser diode current is to be proportional tothe inverted differentiation of the photodiode signal, but thephotodiode signal is subjected to an undesired phase shift by thecircuit. Since the fiber laser requires a performance check at least upto a limiting frequenzy of 500 kHz, the individual components of thecircuit (photodiode with load resistor, amplifier, differentiator) arepreferably designed for a limiting frequency of approximately 10 MHz.

[0033] Test measurements, during which the photodiode was illuminated bya modulated light-emitting diode, yielded the following results: At a500 kHz frequency of the performance fluctuations the circuit produced aphase shift of approximately 25°, at lower frequencies correspondinglysmaller shifts were produced.

[0034] The fiber laser according to the invention is in particularsuitable for use as a light source for confocal microscopes, inparticular a fluorescence microscope.

[0035] Such microscopes are particularly well suited for high-capacityscreening of chemical and/or biological samples.

[0036] Hereunder the invention is explained in detail on the basis ofpreferred embodiments with reference to the drawings in which:

[0037]FIG. 1 shows a schematic view of a basic setup of a preferredembodiment of the fiber laser according to the invention,

[0038]FIG. 2 shows a schematic representation of a preferred embodimentof the optical decoupling means according to the invention,

[0039]FIGS. 3a and 3 b show diagrams of the wavelength versus thereflection of a coupling-in mirror and a coupling-out mirror,respectively,

[0040]FIG. 4 shows a schematic representation of adjustment problemsoccurring during attachment of a mirror to a fiber end,

[0041]FIG. 5 shows a schematic representation of a setup for adjusting amirror at a fiber end,

[0042]FIG. 6 shows a term diagram of praseodym in an host materialZBLAN, and

[0043]FIG. 7 shows a schematic representation of a coupling-out unit.

[0044] The fiber laser according to the invention comprises a pump lightsource 10 which is preferably a laser diode. The light emitted by thepump light source 10 is coupled by a lens 12 into an entrance end 14 ofa fiber 16. For this purpose, a coupling-in mirror 18 is provided at theentrance end 14, said mirror preferably being glued to the fiber end.

[0045] The laser light coupled into the fiber 16 excites the ionscontained in the fiber 16 such that said ions emit laser light with thedesired wavelength. The laser light emerges from the fiber at an exitend 20 of the fiber 16 and can be coupled out by a semi-transmittingmirror 22 in the direction indicated by arrow 24 or by a mirror 28 inthe direction indicated by arrow 25.

[0046] According to the invention, a resonator unit is provided viawhich a portion of the light emerging from the exit end 20 of the fiber16 is fed back into the fiber 16. For this purpose, the resonator unitcomprises a first resonator mirror 26 which is preferably glued to theexit end 20 of the fiber 16. The first resonator mirror 26 is highlyreflecting for the wavelength range of the laser light to be generatedand in the range of the pump light. By back-reflection of the pump lightback into, the fiber 16 the ions in the fiber 16 are additionallyexcited. This increases the efficiency of the fiber laser. Light ofother wavelengths which is not reflected by the first resonator mirror26 can be reflected by the second resonator mirror 28 which ispreferably reflecting for another wavelength range provided for thegeneration of laser light. The light reflected by the second resonatormirror 28 is coupled into the exit end 20 of the fiber 16 by acollimating lens 30.

[0047] A diaphragm 34 shiftable in the direction indicated by arrow 32or a color filter makes it possible to determine how much light of thesecond desired wavelength range is to be fed back into the fiber 16.Shifting of the color diaphragm 34 can be effected by suitable electricand/or magnetic drive units.

[0048] Since deflection of the light by the lens 30 depends on thewavelength, it is also possible to define, by shifting the lens 30 inthe direction indicated by arrow 36, which wavelength range is to becoupled into the exit end 20 of the fiber 16.

[0049] An optical decoupling means 40 shown in FIG. 2 is arrangedbetween the lens 12 and the pump light source 10. The optical decouplingmeans 40 serves for preventing back-reflected light beams 42 having thesame wavelength as the pump light source 10 from being reflected backinto the pump light source 10 and causing disturbances there, e.g. dueto adjustment errors, defects in the surface of the mirror 18 and thelike.

[0050] For this purpose, the decoupling means 40 comprises a polarizingfilter 44 arranged downstream of the pump light source as seen in thedirection of a light beam 46 emitted by the pump light source 10. Thepolarizing filter 44 is configured such that the light 46 emitted by thepump light source 10 can unimpededly pass through the polarizing filter44. Thereafter the light 46 passes through a polarization-changing means48, such as a k/4 plate, which circularly polarizes the linearlypolarized light 46 emitted by the pump light source 10. A reflectedcircularly polarized beam 42 passes through the λ/4 plate 48 again thusbeing linearly polarized. Consequently, the light beam 50 can no longerpass through the polarizing filter 44.

[0051] Below a preferred arrangement of the first resonator mirror 26and the coupling-in mirror 18, respectively, at the exit end 20 and theentrance end 14, respectively, of the fiber 16 is explained in detailwith reference to FIGS. 3-6.

[0052] For producing the mirrors 18,26 preferably a 0.25 mm-0.5 mm thickslab glass of a high surface quality is used as a substrate. In acommercial evaporation coating plant the substrate is coated with the“hard” layers preferably of HfO₂ and SiO₂.

[0053] In the case of direct aluminizing of the fiber the dielectriclayer system is disposed between the glass of the fiber and the ambientair. When the mirror is glued on according to the invention, the layersystem is located between the fiber glass and the slab glass. In thiscase, different layer systems are required than in the case of directaluminizing. In addition to the laser mirror, which bears upon thepolished surface, an antireflection-coating of a thickness of e.g. 850nm is evaporated onto the other substrate side to eliminate disturbingback-reflections.

[0054] By way of example, a coupling-in mirror with a 0.35% transmissionat 491 mm and a coupling-out mirror with a 10% transmission at 491 nmare described below:

[0055]FIGS. 3a and 3 b show the calculated reflection curves of the twomirrors; the following table shows the series of dielectric layers.

27-Layer SiO₂/HfO₂ C upling-in Mirr r for Pr, Yb: ZBLAN Fiber Laser

[0056] No. Material Layer thick. Ref. waveleng. — Glas_(n = 1.5) — —  1SiO₂ λ/4 478  2 HfO₂ λ/8 478  3 SiO₂ λ/4 478  4 HfO₂ λ/4 478  5 SiO₂ λ/4478  6 HfO₂ λ/4 478  7 SiO₂ λ/4 478  8 HfO₂ λ/4 500  9 SiO₂ λ/4 500 10HfO₂ λ/4 500 11 SiO₂ λ/4 500 12 HfO₂ λ/4 500 13 SiO₂ λ/4 500 14 HfO₂ λ/4500 15 SiO₂ λ/4 500 16 HfO₂ λ/4 500 17 SiO₂ λ/4 500 18 HfO₂ λ/4 500 19SiO₂ λ/4 500 20 HfO₂ λ/4 500 21 SiO₂ λ/4 478 22 HfO₂ λ/4 478 23 SiO₂ λ/4478 24 HfO₂ λ/4 478 25 SiO₂ λ/4 478 26 HfO₂ λ/8 478 27 SiO₂ λ/4 478 —Fib. (n = 1.5)

21-Layer SiO₂/HfO₂ Coupling-Out Mirror for Pr, Yb: ZBLAN Fiber Laser

[0057] No. Material Layer thickn. Ref. wavelength — Glas. (n = 1.5) — — 1 HfO₂ λ/4 967  2 SiO₂ λ/4 967  3 HfO₂ λ/4 834  4 SiO₂ λ/4 834  5 HfO₂λ/4 911  6 SiO₂ λ/4 911  7 HfO₂ λ/4 867  8 SiO₂ λ/4 867  9 HfO₂ λ/4 46710 SiO₂ λ/4 467 11 HfO₂ λ/4 489 12 SiO₂ λ/4 489 13 HfO₂ λ/4 489 14 SiO₂λ/4 489 15 HfO₂ λ/4 500 16 SiO₂ λ/4 500 17 HfO₂ λ/4 500 18 SiO₂ λ/4 50019 HfO₂ λ/4 444 20 SiO₂ λ/4 444 21 HfO₂ 0.631*λ/4 444 (22) Air 0 to 3 μm— — Fib. (n = 1.5)

[0058] The reflection curve of the layer systems meets the requirementsfor blue laser operation. The coupling-in mirror displays a reflectivityof 99.65% at 491 nm and a low reflectivity at 570-900 nm.

[0059] The coupling-out mirror displays a reflectivity of 90% at 491 nm,a reflectivity of approximately 70% at 850 nm and a low reflectivity at590-700 nm. An air gap (layer 22) changes the spectral reflection curveas a function of its thickness (FIG. 3b). At a thickness of 75 nm thereflectivity at 635 nm is approximately 7% and at 150 nm approximately14%. Another increase in the air gap thickness causes a decrease and anincrease in the reflectivity. Adjustment of the air gap thickness thusallows emission of laser light in the desired spectral range.

[0060] From the coated substrates having a diameter of 25 mm numeroussmall laser mirrors can be produced. One mirror each is attached to thefiber plug, adjusted and fixed.

[0061] The described concept combines the advantages offered by directevaporation with those offered by exchangeable external aluminizing: Thealuminized fiber is a compact unit which is easy to integrate intooptical setups. On the other hand, the mirrors are easy to exchange,which is of advantage if e.g. the output power of the fiber laser is tobe optimized. The spectral properties of all mirrors produced from oneand the same substrate are identical.

[0062] Methods for precise adjustment and fixing of the small mirrors tothe fiber plug have been developed and are described below.

[0063] In a fiber laser having mirrors glued to the fiber end facesundesired losses occur if the mirrors do not perfectly bear upon thefiber. For optimum feedback, the light reflected back to the fiber musthave the same beam diameter and the same position and beam axis as theoriginal fiber mode. Hereunder, the losses which occur as a consequenceof two types of maladjustments are assessed.

[0064] If a gap of a thickness d exists between the fiber 16 and themirror 18 or 26, or if the mirror is tilted relative to the fiber endface (FIG. 4), losses occur since the emerging beam expands and can nolonger be completely coupled back into the core. If the air gap d (FIG.4) between the fiber 16 and the mirror 26 is e.g. one micrometer, lossesof approximately 3% occur. If the mirror is further tilted at an angleφ=10 relative to the fiber 16, the additional losses amount to 6%. Ifthe losses are to remain in the magnitude of 1%, a spacing of 0.5 μm anda tilt of 0.4° may not be exceeded. If the emission spectrum of thelaser is to be set by selecting the air gap thickness, the preferredsetting range is 0-0.5 μm.

[0065] The hard layers evaporated onto the mirror withstand the highpower densities (>100 MW/cm² at 491 nm and 850 nm). Degradation andshrinking processes which change the reflection behaviour are notobserved.

[0066] The undesired change in the spectral reflection curve caused bythe air gap can be eliminated by placing immersion oil (n≈1.5) into thegap.

[0067] The residual inaccuracy of the adjustment of the mirror spacingshould lie in the magnitude of the wavelength of the visible light. Therequired accuray can be attained by utilizing effects of interferencewith visible light. These effects help to recognize structures which aresmaller than the wavelength of the light used.

[0068] For example, interferences of the light reflected by the mirrorsurface and the fiber end face (60 and 61, respectively), the so-called“Newton's rings” can be used to minimize the tilt and the spacingbetween mirror and fiber axis.

[0069] For very small gap thicknesss destructive interferences existsuch that in the case of supported mirror and vertical viewing, theinterference pattern has a minimum of zero order at the supportlocation. Since for a constructive interference the path difference mustbe an integer multiple of half the wavelength, the gap thickness in themaximum of the first bright circular interference fringe amounts to justλ/4. Thus the interference pattern allows good evaluation of the gapthickness. If the plate is tilted, the support location and thus theposition of the ring system changes. In the case of known radius ofcurvature of the convex surface the tilt can also be assessed.

[0070] For utilizing the Newton's rings the following preferred setuphas been developed: The interference structure occurs between thepolished slightly convex surface of a fiber plug 52 containing the fiber16 and the dielectric mirror evaporated onto a plane glass substrate 26.

[0071] The fiber plug 52 is fixed in a mirror holder which allowsangular adjustment. Onto the polished surface 56 of the plug 52 a smallmirror fragment 26 is placed. The glass plate 54 which is mounted to anx-y-z shifting table is used to press the mirror 26 in position. Thesupport location of the mirror 26 on the plug 52 is adjusted by tiltingthe plug 52. The adjustment work is checked by observing theinterferences with a binocular microscope which is directed to the frontside of the fiber plug 52. The dashed arrows 60 represent light emittedin the direction of the microscope, and the arrows shown as continuouslines 62 represent the light from an illumination means. The contrast ofthe developing interference pattern depends on the type of incidentlight and the reflecting properties of the interfaces. For attaining ahigh contrast of the interference pattern the two interfaces mustdisplay equivalent reflecting properties and the light must bemonochromatic.

[0072] For adjustment purposes preferably two light sources are used:For the adjustment work the quasi-monochromatic light of a sodium vaporlamp is used. Coupled into a flexible glass fiber bundle, the light canbe guided such that the fiber end face is almost vertically illuminated.A halogen lamp serves for illumination of the setup and localization ofthe glass fiber in the ceramic plug.

[0073] When the mirror 26 is adjusted, it is glued to the plug 52.

[0074] Hereunder the advantages of the use of praseodym as the rareearth added to the fiber 16 are explained in detail with reference toFIG. 6.

[0075] The trivalent praseodym ion in the host material ZBLAN offers 12laser transitions in the visible range. The multitude of the colorsmakes praseodym an extremely preferred ion for laser emission in aplurality of spectral ranges, the more so as besides monochrome emissiona switchable and synchronous polychromatic emission is also possible. InFIG. 6 the energy levels and visible transitions of praseodym in ZBLANare plotted: All visible transitions start from the two thermallycoupled levels ³P0 and ^(3P) ₁.

[0076]FIG. 7 shows how two separate control signals for the emission inthe two colors can be derived from the dichromatic radiation of thefiber laser. For this purpose, a spherical lens 30 and an externalfeedback mirror 28 are provided on the coupling-out side, i.e. shown inFIG. 8 on the right of the coupling-out mirror 26. A collimated laserbeam passes through the feedback mirror 28, said laser beam impinging ona split lens 70. The split lens produces three ray bundles 72,74,76. Theray bundle 72 impinges onto a color filter 78 which is blue in theembodiment described. The portion of the ray bundle 72 passing throughthe color filter impinges onto a photodiode 80. Correspondingly, the raybundel 76 impinges onto a color filter 82 which is red in the embodimentdescribed, and the portion of the ray bundle 76 passing through thecolor filter 82 impinges onto a photodiode 84.

1. Fiber laser, comprising: a fiber (16) for laser light generationhaving an entrance end (14) and an exit end (20), a pump light source(10) for generating pump light provided to be coupled via the entranceend (14) into the fiber (16), and a resonator unit (26,28) provided atthe exit end (20) of the fiber, characterized in that the resonator unitcomprises a first resonator mirror (26) connected with the exit end(20), and a second resonator mirror (28) arranged in spaced relationshipto the exit end (20), said second resonator mirror (28) being adaptedfor feeding light of at least one wavelength range emerging at the exitend (20) back into the fiber.
 2. Fiber laser according to claim 1,characterized in that the first resonator mirror (26) is highlyreflecting for the laser light to be generated, namely in the wavelengthrange with the smallest light amplification, offering in particular areflectivity of 50%-99.9%, preferably 80%-99.1%, such that 0.1%-50%,preferably 0.9%-20%, of the generated laser light in this wavelengthrange are coupled out.
 3. Fiber laser according to claim 1 or 2,characterized in that the first resonator mirror (26) is highlyreflecting for the wavelength range of the pump light, offering inparticular a reflectivity of more than 50%, in a particularly preferredaspect of more than 75%.
 4. Fiber laser according to one of claims 1-3,characterized in that an air gap with a thickness of up to 5 μm,preferably up to 3 μm, is provided between the fiber end and the firstresonator mirror (26), said air gap being adjustable and controllableand its thickness determining the wavelength of the light emission. 5.Fiber laser according to one of claims 1-4, characterized in that thesecond resonator mirror (28) is highly reflecting for at least onefurther wavelength range to which the first resonator mirror (26) istessentially transparent such that laser light is generated in thisfurther wavelength range.
 6. Fiber laser according to claim 5,characterized in that the second resonator mirror (28) is tiltedrelative to the optical axis such that light is simultaneously orseparately generated in at least two wavelength ranges.
 7. Fiber laseraccording to one of claims 1-6, characterized in that a diaphragm (34)and/or a color filter for controlling the wavelength range of the laserlight to be generated is arranged upstream of the second resonatormirror (28).
 8. Fiber laser according to claim 7, characterized in thatthe diaphragm and/or a color filter (34) are controllable such thatlaser light is simultaneuosly or separately generated in at least twowavelength ranges.
 9. Fiber laser according to one of claims 4-8,characterized in that the air gap is controllable such that laser lightis simultaneously or separately generated in at least two wavelengthranges.
 10. Fiber laser according to one of claims 5-9, characterized inthat the ratio of the light power in the at least two wavelength rangesis adjustable.
 11. Fiber laser according to one of claims 1-10,characterized in that the resonator unit (26,28) comprises at least onefurther resonator mirror arranged in the beam path behind the secondresonator mirror (28).
 12. Fiber laser according to claim 11,characterized in that the at least one further resonator mirror reflectsa wavelength range to which the resonator mirror or the resonatormirrors arranged in the beam path upstream of the first resonator mirrorare essentially transparent.
 13. Fiber laser according to claim 11 or12, characterized in that a diaphragm and/or a color filter forcontrolling the wavelength range of the laser light to be generated isassociated with each further resonator mirror.
 14. Fiber laser accordingto one of claims 1-13, characterized in that the resonator unitcomprises a shiftable collimating lens (30) for adjusting the wavelengthrange fed back into the fiber.
 15. Fiber laser according to claim 13 or14, characterized in that the collimating lens (30) is configured suchthat it serves for controlling the emission spectrum.
 16. Fiber laseraccording to one of claims 1-15, characterized in that at the entranceend (14) of the fiber (16) a coupling-in mirror (18) transparent to thewavelength range of the pump light is provided.
 17. Fiber laseraccording to claim 16, characterized in that the coupling-in mirror (18)is highly reflecting for the at least one emission wavelength range ofthe fiber.
 18. Fiber laser according to one of claims 1-17,characterized in that an optical decoupling means (40) is providedbetween the pump light source (10) and the entrance end (14) of thefiber (16).
 19. Fiber laser according to claim 18, characterized in thatthe optical decoupling unit (40) comprises a polarizing means.
 20. Fiberlaser according to claim 19, characterized in that the polarizing meanscomprises a polarizing filter (44) arranged downstream of the pump lightsource, the polarizing filter (44) being transparent to the pump lightemitted by the pump light source (10), and a polarization-changing means(48) arranged downstream of the polarizing filter (44) such that pumplight emerging from the entrance end (14) of the fiber (16) or pumplight reflected near the entrance end (14) is polarized such that cannotpass through the polarization filter (44).
 21. Fiber laser according toclaim 17, characterized in that a Faraday rotator or a λ/4 plate isprovided as the polarization-changing means (48).
 22. Fiber laseraccording to one of claims 1-21, characterized in that the fiber ends(14,20) are enclosed by a material with similar hardness and have beenpolished jointly with said material.
 23. Fiber laser according to one ofclaims 1-22, characterized in that the enclosing material is a plasticmaterial polymerizing when exposed to UV-radiation.
 24. Fiber laseraccording to one of claims 1-20, characterized in that the fiber ends(14,20) are produced by cutting.
 25. Fiber laser according to one ofclaims 1-24, characterized in that the first resonator mirror (26)and/or the coupling-in mirror (18) are glued to the exit end (20) andthe entrance end (14), respectively.
 26. Fiber laser according to claim25, characterized in that glueing is effected with a low-viscosityadhesive.
 27. Fiber laser according to claim 25 or 26, characterized inthat a two-component epoxy resin adhesive is used as adhesive.
 28. Fiberlaser according to one of claims 1-24, characterized in that the fiberend faces are prepared as mirrors by direct coating.
 29. Fiber laseraccording to one of claims 1-28, characterized in that the mirrors(18,26,28) are multilayered dielectric mirrors.
 30. Fiber laseraccording to claim 29, characterized in that the layers comprise twodifferent and alternately arranged materials.
 31. Fiber laser accordingto claim 29, characterized in that one layer comprises hafnium oxide andthe other layer comprises silicon oxide.
 32. Fiber laser according toone of claims 27-31, characterized in that the first resonator mirror(26) comprises two multilayer subsystems reflecting the laser lightand/or the pump light.
 33. Fiber laser according to one of claims 1-32,characterized in that the fiber (16) comprises silicon oxide or ZBLANglass.
 34. Fibre laser according to one of claims 1-32, characterized inthat the fiber (16) is doped with at least one rare earth, preferablypraseodym and/or ytterbium and/or erbium ions.
 35. Fiber laser accordingto claim 34, characterized in that the fiber (16) is doped with500-5,000 pppm, preferably 2,000-3,000 ppm, praseodym ions.
 36. Fiberlaser according to claim 34, characterized in that the fiber (16) isdoped with 5,000-50,000 ppm, preferably 10,000-30,000 ppm, Ytterbiumions.
 37. Fiber laser according to one of claims 1-36, characterized inthat the cutoff wavelength for higher waveguide modes of the fiber (16)is by 5%-15% larger than the shortest laser emission wavelength. 38.Fiber laser according to one of claims 1-37, characterized in that thefiber (16) has a length of 10-60 cm, preferably 20-40 cm.
 39. Fiberlaser according to one of claims 1-38, characterized in that from theintensity of the emission power a control signal is generated whichcontrols the emission power of the fiber laser by energizing the pumplight source (10).
 40. Fiber laser according to claim 33, characterizedin that different control signals are generated from the intensities ofthe light power simultaneously emitted at different wavelengths. 41.Fiber laser according to claim 39, characterized in that the intensitiesof the simultaneously emitted light power are determined by coupling outlight at the corresponding wavelength with the aid of a split lens. 42.Fiber laser according to claim 39, characterized in that the controlsignal for the pump light contains a portion proportional to thenegative of the time deflection of the laser output power such thatrapid natural oscillations of the laser light are dampened.
 43. Fiberlaser according to claim 39, characterized in that the control signalfor the pump light contains portions proportional to the deviation ofthe laser power from a nominal value and/or to the time integral of saiddeviation.
 44. Fiber laser according to one of claims 1-43,characterized in that the gap (d) between fiber end face (14,20) andglued-on mirror (18,26) is filled with a medium having an index ofrefraction near that of the fiber.
 45. Fiber laser according to one ofclaims 1-44, characterized in that the fiber (16) comprises a highnumerical aperture, in particular between 0.25 and 0.50.
 46. Fiber laseraccording to one of claims 1-45, characterized in that the secondresonator mirror (28) is exchangeable for the purpose of tuning thefiber laser.
 47. Use of the fiber laser according to one of claims 1-46as a light source for a confocal microscope, in particular for afluorescent microscope.